The present invention relates to production of pulp useful in the manufacture of paper.
Wood represents about 30-45% of total pulp production costs. Thus, increasing the yield of wood conversion into pulp, i.e. the percentage of the wood fed to a pulping operation that usefully becomes part of the pulp solids, is an effective way of achieving the desirable goal of decreasing overall pulp manufacturing costs, by decreasing wood consumption.
Increasing pulp yield provides other benefits such as increased pulp mill through-put and decreased load of black liquor solids to recover. As a consequence there occurs a debottlenecking of the kraft recovery system.
Another important factor in pulp manufacturing costs is the bleaching chemical cost, that can represent 15-20% of the total costs. Total bleach chemical consumption is influenced by the so-called pulp bleachability which is defined as the bleach chemical requirement to achieve a given level of final pulp brightness (e.g. 90% ISO brightness). Thus, increasing pulp bleachabllity during the manufacturing process results in decreased bleach chemical costs.
Those investigating in this field have made many attempts to increase pulp yield and bleachability. Most of the processes proposed are either effective to increase yield or to increase bleachability, but not both at the same time. As a matter of fact, many of the alternate methods proposed to increase pulp yield result in decreased pulp bleachability.
Prior attempts by others with regard to improving yield in the kraft pulp industry can be divided basically into two categories.
One category is processes that use additives together with the wood pulping chemicals to enhance pulping efficiency and to protect pulp carbohydrates. These include: (1) the so-called kraft-anthraquinone pulping whereby anthraquinone (AQ) or any of its derivatives are added together with the pulping chemicals during wood pulping. Besides enhancing the rate of removal from the pulp of the undesirable fractions (lignins) present in the wood, AQ has the property of protecting the desirable fraction (carbohydrates) present in the wood. Anthraquinone acts by oxidation of the carbohydrate reducing end groups, thus increasing overall pulp yield. (2) The so-called kraft-polysulfide pulping whereby polysulfides (PS) are added together with the pulping chemicals to protect the wood carbohydrate fraction. Purportedly, PS have the ability of oxidizing carbohydrate reducing end groups, thus avoiding the so-called xe2x80x9cpeeling reactionxe2x80x9d promoted by the alkali and increasing process yield. (3) The so-called kraft-anthraquinone-polysufide pulping whereby AQ and PS are added together with the wood pulping chemicals. The benefits of these two additives in improving pulp yield have been considered to be synergistic.
It should be noted that the kraft-AO, kraft-PS and kraft-PS-AO processes are effective in improving pulping yield but they have no reported positive effect on pulp bleachability.
In a second category, yield improvements have been based in a more accurate control of kraft pulping kinetics, so that the losses of carbohydrates through peeling reactions and hemicellulose dissolution are minimized. The processes developed for this purpose, the so-called extended delignification processes or modified kraft cooking processes, include among others the isothermal cooking (ITC(copyright)), the low solids cooking (Lo-solids(copyright)), the extended modified continuous cooking (EMCC(copyright)), the rapid displacement heating cooking (RDH(copyright)xe2x80x3), and the Super-Batch(copyright) cooking processes. The four basic principles that are used more or less extensively in these processes are: (1) a constant temperature profile throughout the cook, (2) a constant alkali profile throughout the cook, (3) a high sulfidity throughout the cook, particularly in the beginning and the end of the cook and (4) a low content of solids in the cooking medium throughout the cook.
It should be noted that all these new processes result in yield improvements in the order of 1-2% at most.
Considering that wood and bleach chemicals have the largest impact on pulp manufacturing costs, and that through-put limitations as well as bottlenecked recovery systems are major problems in modern pulp mills, there remains a need for a method to increase pulp yield and bleachability in the manufacturing process so that overall pulp production cost is decreased.
One aspect of the invention is a method for producing pulp, comprising
digesting lignocellulosic wood, containing one or more xylan derivatives selected from the group consisting of xylan bound with lignin, xylan bound with hexenuronic acid, and mixtures thereof, with an aqueous alkaline pulping solution containing sulfide and having an initial free hydroxyl ion concentration of at least 1 mole per liter, under conditions whereunder xylan is dissociated from said one or more xylan derivatives, and the pH of said solution remains above 12.5; and then while the pH of said solution is above 12.5, adding a sufficient amount of an acidic agent to said pulping solution to precipitate dissociated xylan from said pulping solution while minimizing precipitation of lignin from said pulping solution.
As used herein, xe2x80x9cxylanxe2x80x9d and xe2x80x9cxylansxe2x80x9d means polysaccharides composed of repeating units of the formula: 
The term xe2x80x9cxylan derivativesxe2x80x9d means one or more polysaccharides wherein xylan having the aforementioned structure is substituted with one or more substituents and particularly with one or more sugars or with a hexenuronic acid, which is a uronic acid. Examples of xylan derivatives include L-arabino-D-xylans, L-arabino-D-glucurono-D-xylans, L-arabino-(4-O-methyl-D-glucurono)-D-xylans, O-acetyl derivatives of any of the foregoing, xylan bound with lignin, and xylan bound with hexenuronic acid (such as wish hexenuronoxylan or 4-deoxy-xcex2-L-threo-hex-4-enopyranosyluronic acid-xylan).
The term xe2x80x9cboundxe2x80x9d is used herein to mean that two entities, such as xylan and lignin, are xe2x80x9cboundxe2x80x9d to each other if they are held together covalently, ionically, by another attractive force, or by being physically engaged with each other such as by intermolecular entanglement. Two entities are considered xe2x80x9cboundxe2x80x9d if the xe2x80x9cboundxe2x80x9d form precipitates under conditions under which one of the entities, unbound, would precipitate and one of them, unbound, would not.
Two entities are considered xe2x80x9cdisassociatedxe2x80x9d if they are no longer xe2x80x9cboundxe2x80x9d; thus, disassociation can occur by cleaving of covalent bonds, by neutralizing ionic or other attractive forces, or by disentangling or otherwise disengaging the two entities.
Wood is a complex raw material comprising four major components: cellulose (45-50%), hemicelluloses (15-30%), lignin (20-30%) and extractives (1-5%). The hemicellulose fraction includes two major groups: xylan and xylan derivatives; and glucomannan and glucomannan derivatives. The xylan derivatives are predominant in the hardwoods (about 20%) and are present in appreciable amounts in the softwoods.
Conventionally, the aim of wood pulping is to remove into the pulping liquid the lignin and the extractive fractions while retaining in the solid fraction as much as possible of the carbohydrates (cellulose and hemicelluloses). Nevertheless, during pulping a significant fraction of the carbohydrates are also dissolved into the pulping liquid and their value to the pulp is lost. The lignin, he extractives and a significant fraction of the carbohydrates go into solution and become part of the so-called black liquor. The non-dissolved carbohydrate fraction remains as pulp fibers. Conventionally, about 50% of the wood is dissolved in the black liquor, of which about 25-30% is lignin and extractives and the balance 20% are carbohydrates. In the case of hardwoods, about 10% of the dissolved carbohydrates are xylan and xylan derivatives. The yield of the pulping process is directly related to the amount of cellulose and hemicelluloses that are retained in the pulping operation.
Wood pulping via the conventional kraft process is carried out with a mixture of sodium hydroxide and sodium sulfide at appropriate proportions. This mixture is the so-called white liquor. Both sodium hydroxide and sodium sulfide are effective in removing wood lignin and extractives but sodium hydroxide also dissolves fractions of the cellulose and hemicelluloses. The extent of removal of these wood components affects the yield of the process.
The high alkalinity of pulping solutions is generally understood to contribute to loss of cellulose and hemicelluloses for at least three reasons, which are: (1) solubilization of hemicelluloses due to the high alkalinity of the white liquor; (2) alkali hydrolysis of cellulose and hemicelluloses chains resulting in reduction of their polymerization degree and solubilization of the low molecular weight fragments; and (3) successive elimination of cellulose and hemicellulose chain end units through the xe2x80x9cpeelingxe2x80x9d reaction, which occurs at the reducing end group of cellulose and hemicellulose chains. After one unit is removed a new reducing end unit is created and the process progresses.
Thus, it is all the more unexpected that the process of this invention succeeds even though it employs conditions that are even more highly alkaline and would therefore be expected to lead to even worse losses of pulp yield.
In the process of this invention the wood is digested at a high alkalinity, corresponding to a free hydroxyl ion (OHxe2x88x92) concentration of at least 1 mole per liner and preferably at least 1.25 moles per liter, from the beginning of the xe2x80x9ccookxe2x80x9d (i.e. the digestion). This high alkalinity (which corresponds to a pH of at least 14) makes the xylan and xylan derivatives soluble in the pulping liquid. Prior art processes in which pulping starts at a lower initial pH than the method of the present invention, necessarily precipitate significant amounts of xylan derivatives such as those containing xylan-lignin and xylan-hexenuronic acid linkages because these substances precipitate from the very beginning, or closer to the beginning, of the digestion. Thus, for a given kappa number, pulps produced the prior way require much higher quantities of chemicals to reach any given brightness degree, i.e., they show poorer bleachability. Xylan-lignin and xylan-hexenuronic acid linkages are hard to oxidize with conventional bleaching chemicals.
If the initial alkalinity corresponds to a free hydroxyl ion concentration of 1 mole per liter or higher, the xylan derivatives remain in solution for sufficient time so that linkages by which a xylan derivative is bound to its substituent (such as xylan-lignin and xylan-hexenuronic acid linkages) are largely hydrolyzed. This hydrolysis is very important to lessen precipitation of xylan derivatives or e.g. lignin and hexenuronic acid into the pulp solids, along with the xylans when the pH is lowered to, or below, the point at which the xylan precipitates.
In this invention, the higher the initial free hydroxyl ion concentration, and the longer the free hydroxyl ion concentration and thus the pH stay high, the longer it takes for the xylan and xylan derivatives to begin to precipitate. Since a key feature of this invention is to start the digestion at a high enough free hydroxyl ion concentration, and to carry out the digestion for a sufficient period of time, that linkages in xylan derivatives such as xylan-lignin and the xylan-hexenuronic acids are cleaved before the pH of the pulping liquid reaches the point at which xylan precipitates, and since as the digestion proceeds the pH of the pulping liquid decreases such that the xylan and xylan derivatives can eventually start to precipitate, it can be helpful to add additional highly alkaline material (such as alkali metal hydroxides and/or sulfides) during the digestion to keep the pH higher, and above the point at which the xylan precipitates, than would otherwise be the case.
The particulars of the kraft cooking process as well as its operational conditions can readily be ascertained and practiced by those skilled in the art. For the purposes of this invention, conventional cooking conditions can be used except for the active alkali which must be higher than usual (at least 20-30% NaOH based on wood weight). Active alkali is defined here as the sum of NaOH and Na2S concentrations present in the cooking liquor and is expressed as NaOH. Alkalinity is preferably provided by sodium hydroxide or potassium hydroxide. The sulfidity of the cooking liquor and the ratio of pulping liquid to wood can be maintained the same as those used in conventional kraft cooking processes. Thus, in general, a sulfide content of about 15 wt. % to 40 wt. % is effective. Sulfide is preferably provided by sodium sulfide but other compounds can be employed such as polysulfides. A ratio of pulping liquid to wood of about 2:1 to 4:1 (by weight) is effective.
The cooking temperature and the reaction time are adjusted so that the desired pulp kappa number is achieved. Generally, during the digestion a temperature on the order of 155 to 180xc2x0 C. is effective, as is a residence time on the order of 1 to 6 hours. Because of the high initial active alkali the reaction temperature and/or time must be reduced to maintain the pulp kappa number at the desired target. Ideally one should adjust the reaction temperature to meet the desired kappa number and maintain fixed the reaction time. Kappa number is a non-dimensional value which indicates the total amount of oxidizable material present in the pulp after cooking. It is used as a reference for the bleach plant operation. Chemically, it is defined as the number of milliliters of 0.1 N potassium permanganate solution which is consumed by 1 gram of bone dry pulp according to standard procedures (an example of such standard procedure is Tappi method TAPPI um 245).
The qualities of the wood chips used as feed material, and of the pulping liquid, can be maintained the same as those used in conventional kraft cooking. Also the type of equipment (digester) required to cook the wood need not be changed.
At the end of the pulping cycle, just before the content of cooking digester is emptied out into a blow tank, the pH of the pulping liquid is lowered to cause xylan precipitation and therefore promote yield enhancement. Carbonic gas is preferred to lower the pH because this gas causes no side effects in a pulp mill, as carbonates are already part of the black liquor cycle and formation of additional carbonates in the black liquor do not pose a problem for the black liquor recovery cycle. The most preferred conditions in this step are those which provide selective precipitation of xylans, in which the pH is lowered as nearly as possible to a pH value that results in xylan precipitation but not lignin precipitation. The exact pH value where xylan precipitation selectively (without lignin precipitation) occurs depends on many variables such as temperature, type of wood fiber, xylan content, etc. In general a pH of about 12.5 is effective. However, pH values under 12.4 must be avoided, otherwise lignin precipitation occurs.
The conditions for CO2 application can vary, depending upon the cooking system used. In general, the CO2 should be applied at the same conditions existing in the digester at the end of the cooking cycle. The temperature should be in the range of 110-150xc2x0 C. and the pressure in the range of 2-8 atm. The amount of CO2 dosed into the reactor will depend on the pH of the pulping liquid and the final pH desired. In general, to reach a final pH of 12.4, the requirement of CO2 will be in the range of 30-40 kg/ton of pulp. The CO2 will be injected at a pressure slightly higher than that existing in the digester at the end of the cooking cycle.
The physical point into which CO2 is injected can be decided in the light of the type of equipment used. In batch reactors (digesters), the CO2 should be injected in the digester blow line. In continuous digesters the CO2 is added to liquors coming from the cooking zone which have high pH values and this liquor is then recycled back to the digester washing zone which contains lower alkali concentration. At this zone which coincides with the end of the cooking cycle the xylans precipitate.
After the acidification of the pulping liquid with CO2 or otherwise to the proper pH value, the contents of the digester are transferred into another vessel such as a blow tank, and the pulp derived from the wood chips is separated out from the black liquor, washed, screened and stored.
This method is carried out prior to any bleaching step, i.e. prior to addition of any compound that provides bleaching or oxidizing conditions in the pulp.
The pulp produced in accordance with this invention can then be bleached. Another major advantage of this process as compared to the prior art is the production of pulp of higher bleachability.
Bleachability is defined as the chemical requirement to bleach the pulp to a given brightness degree. The higher the bleachability of a given pulp the lower the quantity of bleaching chemicals needed to reach a given brightness degree (for example 90% ISO brightness). Due to the low content: of xylan-lignin and xylan-hexenuronic acid derivatives in pulp produced by the present invention, a smaller quantity of bleaching chemicals is required in order to reach a given brightness degree. In the process of this invention, bleaching chemical savings on the order of 10-15% have been shown in relation to a reference.
Without intending to be bound by any particular theory for the efficacy of the present invention, the tendency of xylan derivatives such as xylan-lignin and xylan-hexenuronic acid compounds to impair pulp bleachability is consistent with the proposition that they contain covalent bonds which are difficult to cleave with conventional bleaching chemicals, and the efficacy of the present invention is consistent with the proposition that this problem is overcome by cleavage of these linkages via alkali hydrolysis in solution in the pulping liquid at the higher pH values described herein, whereupon the xylans that precipitate back onto the fibers in the pulp solids are unsubstituted xylans that do not need to be bleached since they are already white.
As compared to the prior art this invention presents the advantages of not requiring the use of any other chemical or enzyme additive to perform the digesting. Also extensive modification of existing cooking equipment is not required to allow for the so-called modified cooking processes (e.g. RDH(copyright), ITC(copyright), Lo-solids(copyright), Super-Batch(copyright), EMCC(copyright), etc.). All that is needed is a point of CO2 injection at the end of the cooking cycle.
Furthermore, the present invention achieves higher yield gains as compared to the prior art, on the order of 3-4%.